1. Field of the Invention
This invention relates to a developer for developing electrostatic images in electrophotography, electrostatic recording, electrostatic printing, and the like. More particularly, the present invention relates to a developer for electrophotography which can be positively charged strongly and uniformly to visualize the negative electrostatic images, thus giving high quality images, in the method of direct or indirect electrophotographic developing method, and also to a developing method by the use thereof.
2. Description of the Prior Art
In the prior art, there have been known a number of electrophotographic processes, as disclosed in U.S. Pat. No. 2,297,691 or others. In general, a photoconductive material is utilized, electric latent images are formed by various means on a photosensitive member, then said latent images are developed by use of a developing powder (hereinafter referred to as toner), which toner image may, if necessary, be copied on a copying material such as paper, and thereafter fixing of the images effected by heat, pressure or a solvent vapor to give a copied product. When having a step for copying toner image, there is usually provided a step for removing residual toner on the photosensitive member.
As the method for visualization of electric latent images by use of toner, there have been known the magnetic brush method as disclosed in U.S. Pat. No. 2,874,063, the cascade method as disclosed in U.S. Pat. No. 2,618,522, the powder cloud method as disclosed in U.S. Pat. No. 2,221,776, the method using conductive magnetic toner as disclosed in U.S. Pat. No. 3,909,258 and the method using various insulating magnetic toners as disclosed in Japanese Patent Publication No. 9475/1966.
As the toner to be applied for these developing methods, there have been used in the prior art fine powders comprising dyes or pigments dispersed in a natural or synthetic resin. For example, a dispersion having coloring agents dispersed in a binder resin such as polystyrene is pulverized into particles with sizes of 1 to 30.mu. for use as a toner. As a magnetic toner, there is employed a toner in which magnetic particles such as of magnetite are incorporated. In case of the system in which so called two-component developer is used, toner is generally used as a mixture with carrier particles such as glass beads, iron powders, etc.
As the positive charge controlling agents to be used for such a dry system developing, there may generally be included, for example, amino compounds, quaternary ammonium compounds and organic dyes, especially basic dyes and salts thereof. Ordinary positive charge controlling agents are benzyldimethyl-hexadecyl ammonium chloride, decyl-trimethyl ammonium chloride, nigrosine base, nigrosine hydrochloride, safranine .gamma., crystal violet, and the like. In particular, nigrosine base and nigrosine hydrocloride are frequently used as positive charge controlling agents. These materials are generally added to thermoplastic resins, dispersed by melting with heating and the mixture is pulverized, followed by, if necessary, adjustment of the particle sizes, before use.
However, the dyes as these charge controlling agents have complicated structures with various properties and are poor in stability. They may also be decomposed or denatured due to decomposition during hot kneading, mechanical shock, friction, changes in temperature and humidity conditions, etc., thereby causing the phenomenon of lowered charge controllability.
Accordingly, when development is performed by use of a toner containing these dyes as charge controlling agent in a copying machine, the dyes are decomposed or deteriorate as the increase of repeated copying times to cause deterioration of toner during prolonged usage.
The dyes as these charge controlling agents can very difficultly be uniformly dispersed in a thermoplastic resin, and hence the toner particles obtained by pulverization have the vital defect of having different quantities of triboelectrification. On account of such a detect, there have conventionally been performed various methods for effecting more uniform dispersion of these dyes in the resin. For example, basic nigrosine dye is used as a salt with a higher fatty acid for improvement of compatibility with thermoplastic resins. But unaltered fatty acid or a dispersed product of the salt will frequently be exposed on the toner surface to cause contamination of carriers or the support for toner, whereby lowering of fluidity of toner, fogging or lowering of image density may be caused. Alternatively, there is adopted the method for improvement of dispersion of these dyes in the resins to crush and mix mechanically the dye powders with resin powders before hot melt kneading. But the inherent poor dispersibility cannot be avoided, and there has not yet been obtained actually sufficient uniformness of charging.
Most of the positive charge controlling dyes are hydrophilic and, due to poor dispersibility into the resins, dyes may be exposed on the surface of toner when pulverized after melt kneading. During usage of said toner under highly humid conditions, hydrophilic property of these dyes makes it difficult to obtain good images.
As described above, in the case of dyes of prior art having positive charge controlling characteristic, there may be caused variance in charge quantities generated on toner particles between toner particles, between toner and carrier or between toner and toner support such as a sleeve, whereby such troubles as developing fog, toner scattering and carrier contamination will occur. These undesirable phenomena will appear more conspicuously when a large number of copying papers are piled, to give a result unsuitable for a copying machine.
Under highly humid conditions, the transferring efficiency of toner image is also markedly lowered and therefore such a toner is no longer useful. Even under normal temperature and normal humidity, when said toner is stored for a long time, toner agglomeration may occur on account of instability of the positive charge controlling dye employed, thus becoming frequently impossible for use.
On the other hand, as the method for fixing toner images on support, there have been invented various fixing techniques in the past by utilization of heat, pressure and solvent. Among them, fixing system by pressurization has advantages of requiring a little amount of energy and dispensing with the waiting time for copying.
According to a pressure fixing system, it is required to use a toner material which is less tacky but greater in cohesive force to have a small frictional coefficient. As such a toner material, ethylenic olefin polymers, typically polyethylene, are well known in the art. At present, however, it is difficult to obtain a toner for developing negative latent image with the use of polyethylene. In the prior art, nigrosine base and nigrosine hydrochloride are frequently used as positive charge controller. While these can act comparatively effectively on toner materials to be used for thermal fixing system, typically polystyrene, to make the toner positively chargeable, they are not effective at all for ethylenic olefin polymers.
To explain in further detail about the developing method as mentioned above, the developing methods may be classified broadly into dry system developing method and wet system developing method. The former is further differentiated into two categories, one being a method using a two-component system developer and the other a method using a one-component system developer. The methods belonging to the two-component system developing method include various methods using various carriers for conveying toner, such as the magnet brush method using iron powder carrier, the cascade method using beads carrier and the fur brush method using fur.
On the other hand, those belonging to the one-component systym method, there are the powder cloud method in which toner particles are used in an atomized state, the contact developing method (or toner developing) in which development is performed by contacting toner particles directly with the electrostatic latent image surface, the jumping method in which toner particles are not contacted directly with the electrostatic latent image surface but toner particles are charged and permitted to flight through the electric field possessed by the electrostatic latent image toward said latent image, and the magne-dry method in which development is performed by contacting magnetic conductive toners on electrostatic latent image surface. According to the two-component system method, a mixed developer of carrier particles and toner particles is necessarily used, and toner particles are consumed in an amount by far greater than carrier particles in the usual progress of development, whereby the mixing ratio of both particles is changed to cause variations in density of images of interest. Another disadvantage inherent in this method is that carrier particles difficultly consumed will be deteriorated on prolonged usage to lower the image quality.
On the other hand, according to the one-component developing method such as the magne-dry method using magnetic toner and the contact developing method using no magnetic toner, toner is contacted on all the surface to be developed, irrespectively of the image portion or the non-image portion. For this reason, there is the problem that toner may be attached also on the non-image portion to cause easily contamination of so called ground fogging. (This fogging contamination was also the disadvantage similarly caused in the two-component system developing method.) Also, in the powder cloud method, attachment of powdery toner particles on the non-image portion cannot be avoided and thus said method has also the disadvantage of being not free from ground fogging.
Further, as the so called jumping method belonging to one-component system developing method, there is known a method in which a support such as a sheet is coated unformly with a toner and faced with a surface holding electrostatic charges at a small gap, and the toner is attracted from the toner support by the charges possessed by the electrostatic image to be attached onto the electrostatic image holding surface (see U.S. Pat. No. 2,839,400).
According to this method, there is the advantage that no toner is attracted onto the non-image portion devoid of electrostatic charges. Still another advantage is that the aforesaid fogging will difficultly occur because toner is not contacted with non-image portion. Further, since no carrier particle is used, there is neither change of the mixing ratio as mentioned above nor deterioration of carrier particles.
However, according to this method, in which electric field is given for attachment of toner previously on a toner supporting sheet, it is difficult to apply uniformly and thinly toner onto the support, and therefore coating irregularity is liable to be formed. As another disadvantage, when the toner layer coated is faced toward the electrostatic image, uniform release of toner toward the electrostatic image is also difficult.
In this respect, the developing device as proposed in Japanese Laid-open Patent Application No. 43027/1979 and No. 18656/1980, having magnetic toner and movable toner support (sleeve roller) and static magnet provided internally of said support, in which there is arranged a member for regulating toner thickness of a magnetic material near the outer surface of the sleeve roller so as to face the magnetic pole of said magnet, enables uniform and thin coating of toner on the outer surface of the sleeve roller. Such device has overcome the drawbacks as mentioned above and may be appreciated as an electrostatic image developing device which can give highly faithful and stable images.
The present inventors have found the problem that image quality is lowered by lowering of image density when copying is continued repeatedly by using a jumping developer known in the art, especially under the conditions of high temperature or high humidity. As the result of studies made on this point, it has been found that one of the causes for such a phenomenon resides in stability and reliability of the charge controlling component, on account of which attachment of developing powders onto said sleeve and transfer of developing powders from the sleeve are changed.